Evidence – it’s all about the context

When we talk about forensic evidence, most people think all we need to do is find a DNA profile and that’s it – job done. It’s usually rather more complicated than that. What we’re really looking for is something that will link the suspect to the offence and hopefully rule out anyone else, and a DNA profile isn’t always the best tool for the job. For instance, imagine you’re examining the scene of a violent mugging in a park, what might help you to identify the perpetrator? Some CCTV footage might be nice, but usually what you get is of the back of someone’s head, or a grainy image their own mother would be hard pressed to recognize.

It’s true that DNA can reliably identify a person from evidence found at the scene – let’s say, from a discarded cigarette end, which is potentially a rich source of DNA. Even if the DNA identifies a likely suspect, Forensic science can only tell you that someone was at a particular place, but not necessarily when they were there. This is often a problem for investigators – you identify someone who may be the offender; or it maybe they just happened to walk past the spot, and dropped a cigarette butt.

I once examined the scene of a house burglary in which the intruder had moved the TV in order to steal the DVD player beneath it. I found fingerprints on the side of the TV, right where you would hold it if you were lifting it up. I recovered the fingerprints and sent them off to the fingerprint bureau, confident that they’d identify the offender. Indeed, a man was quickly identified and brought in for questioning, but he denied all knowledge of the burglary. Well, he would, wouldn’t he? Although to be fair he didn’t really fit the stereotype of a burglar: he was employed, and his fingerprints were only on record because he’d been arrested following a drunken fight fifteen years earlier.

So he was either a master criminal who’d evaded capture for years before inexplicably becoming sloppy – or something wasn’t quite right …

It turned out that he was a delivery driver for a high street electrical shop and had actually delivered and installed the TV several years earlier, so he had been at the scene of the crime – just not when the crime was committed . . . It’s a pity fingerprints don’t come with a date stamp.

So back to our mugging– where are we going to find some useful forensic evidence?

It wouldn’t surprise me if there wasn’t anything at all at the actual scene. Hopefully a police dog would establish a track to follow the way the offender made off after the attack. Generally, offenders discard anything not valuable to them or which might link them to the offence, so it’s likely that the stolen bag / wallet will be recovered, largely intact, nearby. The offender will have rifled through, primarily looking for (untraceable) cash and maybe a mobile phone. This is good news for us, because even if the offender wore gloves during the attack, it’s almost impossible to sort through a wallet with gloved fingers. So in this case my money would be on finding some fingerprints on the discarded property – but I’d have my fingers crossed that they didn’t belong to someone who worked in a bag shop!